DY – Rambam Shechitah 10:12
? ???? ?????? ?? ?????? ???, ???: ??? ????? ????? ?? ???? ?? ???? ??? ???? ???? ???? ?????? ????????, ??????? ????? ??? ???? ?????–???? ?????. ?????? ???? ??? ???? ??????, ???? ???? ?????. [??] ??? ??? ???? ????? ??? ?????–?? ?? ?? ?????? ????? ?????? ??????? ?????? ???? ??????, ????? ????? ???: ??? ?? ??? ?? ???? ?????, ????? “?? ?? ????? ??? ?????” (????? ??,??).
So the actual Meztius is not relevant, rather it is what Chazal decided to be the Halacha that defines the Halachic Metzius.
So when you say
“I’m saying we do know how they affect the din – it’s only muttar if and because they are ???? ??? ????. We may not know which physical characteristic is considered ???? ??? ????.”
I’m understanding the “physical characteristic” as the Halachic difference that gives the item (louse or animal) its Din.